The present invention relates generally to novel electrically conductive adherent and coherent film forming adhesives, and more particularly to such an electrically conductive film forming adhesive which rapidly cures from a flowable liquid state to an adherent and coherent solid state at modest temperatures such as temperatures below about 120.degree. C. The conductive adhesive formulations of the present invention are particularly adapted for use in the direct bonding of various active or passive components to substrates having printed circuits thereon, particularly flexible printed circuits fabricated from heat sensitive substrates such as biaxially oriented polyethylene terephthalate.
In the past, adhesives, including conductive adhesives, have been widely employed in the bonding of electrical and electronic components to temperature sensitive substrates. Typically, such adhesives have been in the form of two-component systems which must be pre-mixed in precise ratios, and must be used within an extremely short period of time because of their limited pot life. Alternatively, single component conductive bonding systems have been proposed and have been available, however such systems are primarily intended for use with substrates capable of exposure to reasonably highly elevated temperatures, since such single component conductive bonding formulations have normally utilized relatively high curing temperatures. The present invention provides a single component system which eliminates the problems created by exposure to high temperatures, with the present system achieving a cure at temperatures below about 120.degree. C., while requiring a cure-cycle of only a relatively few minutes.
Conductive bonding agents are in wide use in the bonding of electronic components, including both active and passive components such as transistors, diodes, light emitting diodes as well as other solid-state components, resistors, capacitors, and the like. For most production processes, it is desirable that the bonding agents undergo a cure cycle in the shortest possible interval of time. One of the most expeditious way of reducing cure time is, of course, the utilization of a higher curing temperature. Such an expedient is not available when dealing with temperature sensitive materials.
Substrates in wide use at the present time include, in particular, biaxially oriented polyethylene terephthalate. The temperature capability of stress oriented polyethylene terephthalate places a relatively low limit upon the temperature which may be employed during the cure cycle for the bonding agents employed, including conductive bonding agents. The limitations imposed upon these systems make it necessary to carefully control the temperatures involved in the cure cycle, as well as the mass of conductive bonding agent employed, bearing in mind that exothermic reactions frequently occur during the cure cycle. The combination of the elevated cure temperature, together with the terminal gain derived form the exothermic reaction, all contribute to the need to control the curing cycle, and furthermore contribute to time extension for the curing cycle involved.
A further consideration in the utilization of conductive bonding agents is the frequent requirement of surface pre-treatment for stress oriented polyethylene terephthalate. In order to provide utility for certain bonding agents, it has frequently been found necessary to treat the surface of stress oriented polyethylene terephthalate with a substance which will enhance the adhesion between the bonding agent and the surface of the film. In accordance with the formulations of the present invention, however, such surface pre-treatments are not required, and untreated surfaces of stress oriented polyethylene terephthalate establish acceptable bonds.